Ink jet printers have become very popular due to their quiet and fast operation and their high print quality on plain paper. A variety of ink jet printing methods have been developed.
In one ink jet printing method, termed continuous jet printing, ink is delivered under pressure to nozzles in a print head to produce continuous jets of ink. Each jet is separated by vibration into a stream of droplets which are charged and electrostatically deflected, either to a printing medium or to a collection gutter for subsequent recirculation. U.S. Pat. No. 3,596,275 is illustrative of this method.
In another ink jet printing method, termed electrostatic pull printing, the ink in the printing nozzles is under zero pressure or low positive pressure and is electrostatically pulled into a stream of droplets. The droplets fly between two pairs of deflecting electrodes that are arranged to control the droplets' direction of flight and their deposition in desired positions on the printing medium. U.S. Pat. No. 3,060,429 is illustrative of this method.
A third class of methods, more popular than the foregoing, is known as drop-on-demand printing. In this technique, ink is held in the pen at below atmospheric pressure and is ejected by a drop generator, one drop at a time, on demand. Two principal ejection mechanisms are used: thermal bubble and piezoelectric pressure wave. In the thermal bubble systems, a thin film resistor in the drop generator is heated and causes sudden vaporization of a small portion of the ink. The rapidly expanding ink vapor displaces ink from the nozzle causing drop ejection. U.S. Pat. No. 4,490,728 is exemplary of such thermal bubble drop-on-demand systems.
In the piezoelectric pressure wave systems, a piezoelectric element is used to abruptly compress a volume of ink in the drop generator, thereby producing a pressure wave which causes ejection of a drop at the nozzle. U.S. Pat. No. 3,832,579 is exemplary of such piezoelectric pressure wave drop-on-demand systems.
The drop-on-demand techniques require that under quiescent conditions the pressure in the ink reservoir be below ambient so that ink is retained in the pen until it is to be ejected. The amount of this "underpressure" (or "partial vacuum") is critical. If the underpressure is too small, or if the reservoir pressure is positive, ink tends to escape through the drop generators. If the underpressure is too large, air may be sucked in through the drop generators under quiescent conditions. (Air is not normally sucked in through the drop generators because their high capillarity retains the air-ink meniscus against the partial vacuum of the reservoir.)
The underpressure required in drop-on-demand printing systems can be obtained in a variety of ways. In one system, the underpressure is obtained gravitationally by lowering the ink reservoir so that the surface of the ink is slightly below the level of the nozzles. However, such positioning of the ink reservoir is not always easily achieved and places severe constraints on print head design. Exemplary of this gravitational underpressure technique is U.S. Pat. No. 3,452,361.
Alternative techniques for achieving the required underpressure are shown in U.S. Pat. No. 4,509,062 and in copending application Ser. No. 07/115,013 filed Oct. 28, 1987, both assigned to the present assignee. In the former patent, the underpressure is achieved by using a bladder type ink reservoir which progressively collapses as ink is drawn therefrom. The restorative force of the flexible bladder keeps the pressure of the ink in the reservoir slightly below ambient. In the system disclosed in the latter patent application, the underpressure is achieved by using a capillary reservoir vent tube that is immersed in ink in the ink reservoir at one end and coupled to an overflow catchbasin open to atmospheric pressure at the other. As the printhead draws ink from the reservoir, the reservoir pressure falls below ambient. This underpressure increases as ink is ejected from the reservoir. When the underpressure reaches a threshold value, it draws a small volume of air in through the capillary tube and into the reservoir, thereby preventing the underpressure from exceeding the threshold value.
The maximum print rate in drop-on-demand printers is limited by the recharge time of the capillary tube that provides ink to the drop generator. If the drop generator is fired more quickly that the capillary tube can supply ink, the droplets comprising the ink jet will be incompletely formed and some may be omitted entirely.
There is a long felt and increasing need for higher speed ink jet printers, especially with for use in print-intensive applications, such as the printing of graphical images. Existing ink jet pens have been optimized to obtain every possible speed advantage, such as by exploitation of the oscillation of the ink in the drop generator to speed the rate at which droplets can be ejected, yet the need for still faster ink jet printers persists.
It is an object of the present invention to fulfill this need.
It is a more particular object of the present invention to provide an ink jet pen that has two modes of operation: a regular speed mode and a high speed mode.
It is another more particular object of the present invention to provide an ink jet pen that can selectably supply ink to the drop generator at either a negative pressure or at a positive pressure.
It is still another more particular object of the present invention to provide an ink jet pen that can automatically close a vent in its ink reservoir so that a positive pressure can be maintained therein.
According to one embodiment of the present invention, an ink jet pen is provided with a electrical heating element that can be selectably energized to heat air in the ink reservoir and thereby increase the pressure on the ink therein. This positive pressure drives the ink more rapidly through the tube feeding the drop generator and permits the pen to print at a faster rate.
When the heating element is not energized, the partial vacuum left in the reservoir by the ejection of ink is moderated by the introduction of air through a bubble generator orifice. This orifice is sized so that a negative reservoir pressure of about 5 inches of water is required before a bubble of air can be drawn through the orifice and into the ink. By this arrangement, the reservoir pressure is regulated at the "bubble pressure" when the heating element is not energized.
The pressure in the reservoir is also regulated when the heating element is energized. The positive pressure in the reservoir would normally tend to drive ink out the bubble generator orifice. In the present invention, however, the ink is prevented from draining out the bubble generator orifice until the reservoir pressure exceeds a positive threshold value. When that pressure is exceeded, a volume of ink is forcibly expelled. This expulsion of ink relieves a portion of the positive pressure in the reservoir and keeps the reservoir pressure below the positive threshold value.
In one embodiment, ink is prevented from draining out the bubble generator orifice when the heating element is energized by a novel arrangement of components in the catchbasin chamber to which the orifice leads. This chamber is vented to the atmosphere through a chimney that extends into the chamber and terminates with its opening opposite the bubble generator orifice. When ink begins to be driven by a positive pressure from the reservoir through the bubble generator orifice and into the chamber, the ink seals the opening in the chimney, thereby isolating the chamber from ambient pressure. Thereafter, positive pressure in the ink reservoir caused by the heating of air therein is relieved by forcing ink to the print nozzles at a faster rate during printing.
If the heating element is not energized and the pressure in the reservoir rises above ambient due to environmental conditions, the above-described ventblocking mechanism is disabled and the positive pressure in the reservoir is relieved by discharging ink to the catchbasin.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.